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43-509: For IPv4 packets, Path MTU Discovery works by setting the Don't Fragment (DF) flag bit in the IP headers of outgoing packets. Then, any device along the path whose MTU is smaller than the packet will drop it, and send back an Internet Control Message Protocol (ICMP) Fragmentation Needed (Type 3, Code 4) message containing its MTU, allowing the source host to reduce its path MTU appropriately. The process

86-430: A Sequence Number that is only reset at boot time. The Echo Reply is returned as: An ICMP packet transported with IPv6 looks like this. Most Linux systems use a unique Identifier for every ping process, and Sequence Number is an increasing number within that process. Windows uses a fixed Identifier , which varies between Windows versions, and a Sequence Number that is only reset at boot time. The Echo Reply

129-441: A standard deviation of 0.748 ms. In cases of no response from the target host, most implementations display either nothing or periodically print notifications about timing out. Possible ping results indicating a problem include the following: In case of error, the target host or an intermediate router sends back an ICMP error message, for example host unreachable or TTL exceeded in transit . In addition, these messages include

172-401: A layer 3 protocol in the modern five-layer TCP/IP protocol definitions (by Kozierok, Comer, Tanenbaum, Forouzan, Kurose, Stallings). There is no TCP or UDP port number associated with ICMP packets as these numbers are associated with the transport layer above. The ICMP packet is encapsulated in an IPv4 packet. The packet consists of header and data sections. The ICMP header starts after

215-454: A part of reconnaissance attack to gather information on the target network, therefore ICMP Address Mask Reply is disabled by default on Cisco IOS. Address mask reply is used to reply to an address mask request message with an appropriate subnet mask. Where: Destination unreachable is generated by the host or its inbound gateway to inform the client that the destination is unreachable for some reason. Reasons for this message may include:

258-466: A single request wakes up that host just enough to allow its Echo Reply service to reply instantly if that service was enabled. The host does not need to wake up all devices completely and may return to low-power mode after a short delay. Such configuration may be used to avoid a host to enter in hibernation state, with much longer wake-up delay, after some time passed in low power active mode. A packet including IP and ICMP headers must not be greater than

301-423: Is a mechanism for routers to convey routing information to hosts. The message informs a host to update its routing information (to send packets on an alternative route). If a host tries to send data through a router (R1) and R1 sends the data on another router (R2) and a direct path from the host to R2 is available (that is, the host and R2 are on the same subnetwork ), then R1 will send a redirect message to inform

344-500: Is carried in one or more Extention Objects, which are preceded by an ICMP Extension Header. Extension objects have the following general structure: Ping (networking utility) ping is a computer network administration software utility used to test the reachability of a host on an Internet Protocol (IP) network. It is available for virtually all operating systems that have networking capability, including most embedded network administration software. Ping measures

387-420: Is configured to accept wakeup requests. If the host is already active and configured to allow replies to incoming ICMP Echo Request packets, the returned reply should include the same payload. This may be used to detect that the remote host was effectively woken up, by repeating a new request after some delay to allow the host to resume its network services. If the host was just sleeping in low power active state,

430-403: Is discarded until the queue is no longer full. But as no acknowledgement mechanism is present in the network layer, the client does not know whether the data has reached the destination successfully. Hence some remedial measures should be taken by the network layer to avoid these kind of situations. These measures are referred to as source quench. In a source quench mechanism, the router sees that

473-445: Is generated by a gateway to inform the source of a discarded datagram due to the time to live field reaching zero. A time exceeded message may also be sent by a host if it fails to reassemble a fragmented datagram within its time limit. Time exceeded messages are used by the traceroute utility to identify gateways on the path between two hosts. Where: Timestamp is used for time synchronization. The originating timestamp

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516-450: Is implemented using the ICMP echo request and echo reply messages. ICMP uses the basic support of IP as if it were a higher-level protocol, however, ICMP is actually an integral part of IP. Although ICMP messages are contained within standard IP packets, ICMP messages are usually processed as a special case, distinguished from normal IP processing. In many cases, it is necessary to inspect

559-400: Is not available or that a host or router could not be reached. ICMP differs from transport protocols such as TCP and UDP in that it is not typically used to exchange data between systems, nor is it regularly employed by end-user network applications (with the exception of some diagnostic tools like ping and traceroute ). A separate Internet Control Message Protocol (called ICMPv6 )

602-507: Is repeated until the MTU is small enough to traverse the entire path without fragmentation. As IPv6 routers do not fragment packets, there is no Don't Fragment option in the IPv6 header . For IPv6, Path MTU Discovery works by initially assuming the path MTU is the same as the MTU on the link layer interface where the traffic originates. Then, similar to IPv4, any device along the path whose MTU

645-402: Is returned as: The payload of the packet is generally filled with ASCII characters, as the output of the tcpdump utility shows in the last 32 bytes of the following example (after the eight-byte ICMP header starting with 0x0800 ): The payload may include a timestamp indicating the time of transmission and a sequence number, which are not found in this example. This allows ping to compute

688-455: Is set to the time (in milliseconds since midnight) the sender last touched the packet. The receive and transmit timestamps are not used. Where: Timestamp Reply replies to a Timestamp message. It consists of the originating timestamp sent by the sender of the Timestamp as well as a receive timestamp indicating when the Timestamp was received and a transmit timestamp indicating when

731-411: Is smaller than the packet will drop the packet and send back an ICMPv6 Packet Too Big (Type 2) message containing its MTU, allowing the source host to reduce its path MTU appropriately. The process is repeated until the MTU is small enough to traverse the entire path without fragmentation. If the path MTU changes after the connection is set up and becomes lower than the previously determined path MTU,

774-565: Is standardized under RFC 8899, Datagram Packetization Layer Path MTU Discovery (DPLPMTUD). Upon loss of connectivity, DPLPMTUD utilizes probe packets of controlled sizes to probe the MTU of the path. Acknowledgement of a probe packet indicates that the path MTU is at least the size of that packet. Usage of DPLPMTUD is standardized in QUIC . However, in order for transport layer protocols to operate most efficiently, ICMP Unreachable messages (type 3) should still be permitted. Some routers, including

817-422: Is the output of running ping on Linux for sending five probes (1-second interval by default, configurable via -i option) to the target host www.example.com : The output lists each probe message and the results obtained. Finally, it lists the statistics of the entire test. In this example, the shortest round-trip time was 9.674 ms, the average was 10.968 ms, and the maximum value was 11.726 ms. The measurement had

860-578: Is used with IPv6 . ICMP is part of the Internet protocol suite as defined in RFC 792. ICMP messages are typically used for diagnostic or control purposes or generated in response to errors in IP operations (as specified in RFC 1122). ICMP errors are directed to the source IP address of the originating packet. For example, every device (such as an intermediate router ) forwarding an IP datagram first decrements

903-465: The IPv4 header and is identified by its protocol number , 1 . All ICMP packets have an eight-byte header and variable-sized data section. The first four bytes of the header have fixed format, while the last four bytes depend on the type and code of the ICMP packet. ICMP error messages contain a data section that includes a copy of the entire IPv4 header, plus at least the first eight bytes of data from

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946-652: The Timestamp reply was sent. Where: The use of Timestamp and Timestamp Reply messages to synchronize the clocks of Internet nodes has largely been replaced by the UDP-based Network Time Protocol and the Precision Time Protocol . Address mask request is normally sent by a host to a router in order to obtain an appropriate subnet mask . Recipients should reply to this message with an Address mask reply message. Where: ICMP Address Mask Request may be used as

989-400: The round-trip time for messages sent from the originating host to a destination computer that are echoed back to the source. The name comes from active sonar terminology that sends a pulse of sound and listens for the echo to detect objects under water. Ping operates by means of Internet Control Message Protocol (ICMP) packets . Pinging involves sending an ICMP echo request to

1032-504: The time to live (TTL) field in the IP header by one. If the resulting TTL is 0, the packet is discarded and an ICMP time exceeded message is sent to the datagram's source address. Many commonly used network utilities are based on ICMP messages. The traceroute command can be implemented by transmitting IP datagrams with specially set IP TTL header fields, and looking for ICMP time exceeded in transit and destination unreachable messages generated in response. The related ping utility

1075-699: The IPv4 packet that caused the error message. The length of ICMP error messages should not exceed 576 bytes. This data is used by the host to match the message to the appropriate process. If a higher level protocol uses port numbers, they are assumed to be in the first eight bytes of the original datagram's data. The variable size of the ICMP packet data section has been exploited . In the " Ping of death ", large or fragmented ICMP packets are used for denial-of-service attacks . ICMP data can also be used to create covert channels for communication. These channels are known as ICMP tunnels . Control messages are identified by

1118-502: The Linux kernel and Cisco, provide an option to reduce the maximum segment size (MSS) advertised in the TCP handshake as a workaround. This is known as MSS clamping . Another problem is when networks administrators don't properly update the MTU between 2 adjacent layer 3 hops if the link between these hops is composed of multiple layer 2 segments with switches between them. Usually the MTU on

1161-526: The contents of the ICMP message and deliver the appropriate error message to the application responsible for transmitting the IP packet that prompted the ICMP message to be sent. ICMP is a network-layer protocol; this makes it a layer 3 protocol in the seven-layer OSI model . Based on the four-layer TCP/IP model, ICMP is an internet-layer protocol, which makes it a layer 2 protocol in the Internet Standard RFC 1122 TCP/IP four-layer model or

1204-486: The errors that are necessary for the proper operation of PMTUD. This can result in connections that complete the TCP three-way handshake correctly but then hang when attempting to transfer data. This state is referred to as a black hole connection . Some implementations of PMTUD attempt to circumvent this problem by inferring that large payload packets have been dropped due to MTU rather than link congestion. One such scheme

1247-458: The first eight bytes of the original message (in this case header of the ICMP echo request, including the quench value), so the ping utility can match responses to originating queries. An ICMP packet transported with IPv4 looks like this. Most Linux systems use a unique Identifier for every ping process, and Sequence Number is an increasing number within that process. Windows uses a fixed Identifier , which varies between Windows versions, and

1290-437: The first large packet will cause an ICMP error and the new, lower path MTU will be found. If the path changes and the new path MTU is larger, the source will not learn about the increase, because all routers along the new path will be capable of relaying all packets that the source sends using the originally determined, lower path MTU. Many network security devices block all ICMP messages for perceived security benefits, including

1333-499: The host that the best route for the destination is via R2. The host should then change its route information and send packets for that destination directly to R2. The router will still send the original datagram to the intended destination. However, if the datagram contains routing information, this message will not be sent even if a better route is available. RFC 1122 states that redirects should only be sent by gateways and should not be sent by Internet hosts. Where: Time Exceeded

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1376-415: The incoming data rate is much faster than the outgoing data rate, and sends an ICMP message to the clients, informing them that they should slow down their data transfer speeds or wait for a certain amount of time before attempting to send more data. When a client receives this message, it automatically slows down the outgoing data rate or waits for a sufficient amount of time, which enables the router to empty

1419-469: The next L3 hop. Internet Control Message Protocol The Internet Control Message Protocol ( ICMP ) is a supporting protocol in the Internet protocol suite . It is used by network devices , including routers , to send error messages and operational information indicating success or failure when communicating with another IP address . For example, an error is indicated when a requested service

1462-759: The number of network hops ( TTL ) that probes traverse, interval between the requests and time to wait for a response. Many systems provide a companion utility ping6, for testing on Internet Protocol version 6 (IPv6) networks, which implement ICMPv6 . The ping utility was written by Mike Muuss in December 1983 during his employment at the Ballistic Research Laboratory , now the US Army Research Laboratory . A remark by David Mills on using ICMP echo packets for IP network diagnosis and measurements prompted Muuss to create

1505-403: The outgoing L3 interface is taken from the first L2 segment. But if the second or further segment has a lower MTU the switch that is between will just silently drop the packet without reporting back any ICMP (because only layer 3 hops can generate ICMP "packet too big"). So, in this case admins should update the MTU for each outgoing L3 interface to the minimum MTU of the layer 2 segments used until

1548-497: The physical connection to the host does not exist (distance is infinite); the indicated protocol or port is not active; the data must be fragmented but the 'don't fragment' flag is on. Unreachable TCP ports notably respond with TCP RST rather than a destination unreachable type 3 as might be expected. Destination unreachable is never reported for IP multicast transmissions. With the following field contents: ICMP messages can be extended with extra information. This information

1591-512: The queue. Thus the source quench ICMP message acts as flow control in the network layer. Since research suggested that "ICMP Source Quench [was] an ineffective (and unfair) antidote for congestion", routers' creation of source quench messages was deprecated in 1995 by RFC 1812. Furthermore, forwarding of and any kind of reaction to (flow control actions) source quench messages was deprecated from 2012 by RFC 6633. Where: Redirect requests data packets be sent on an alternative route. ICMP Redirect

1634-475: The round-trip time in a stateless manner without needing to record the time of transmission of each packet. The payload may also include a magic packet for the Wake-on-LAN protocol, but the minimum payload, in that case, is longer than shown. The Echo Request typically does not receive any reply if the host was sleeping in hibernation state, but the host still wakes up from sleep state if its interface

1677-411: The router or host buffer is approaching its limit. Data is sent at a very high speed from a host or from several hosts at the same time to a particular router on a network. Although a router has buffering capabilities, the buffering is limited to within a specified range. The router cannot queue any more data than the capacity of the limited buffering space. Thus if the queue gets filled up, incoming data

1720-426: The target host and waiting for an ICMP echo reply . The program reports errors, packet loss , and a statistical summary of the results, typically including the minimum, maximum, the mean round-trip times, and standard deviation of the mean. The command-line options of the ping utility and its output vary between the numerous implementations. Options may include the size of the payload, count of tests, limits for

1763-413: The utility to troubleshoot network problems. The author named it after the sound that sonar makes since its methodology is analogous to sonar's echolocation. The backronym Packet InterNet Groper for PING has been used for over 30 years, and although Muuss says that from his point of view, PING was not intended as an acronym, he has acknowledged Mills' expansion of the name. The first released version

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1806-414: The value in the type field. The code field gives additional context information for the message. Some control messages have been deprecated since the protocol was first introduced. Source Quench requests that the sender decrease the rate of messages sent to a router or host. This message may be generated if a router or host does not have sufficient buffer space to process the request, or may occur if

1849-612: Was public domain software ; all subsequent versions have been licensed under the BSD license . Ping was first included in 4.3BSD . The FreeDOS version was developed by Erick Engelke and is licensed under the GPL . Tim Crawford developed the ReactOS version. It is licensed under the MIT License . Any host must process ICMP echo requests and issue echo replies in return. The following

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